The invention relates to a moisture-crosslinkable polyurethane adhesive which can be used in particular in the field of hygiene.
It is now commonplace to use fixing systems of the VELCRO(copyright) type to prepare disposable articles relating to the field of hygiene, in particular diapers for babies or incontinent adults and feminine hygiene articles. It is well known that the systems of the above-mentioned type contain two components: a first component composed of a multitude of fine hooks and a second component composed of a multitude of fine loops. By simply pressing on the superimposed components, the hooks are made to enter the loops, which makes it possible to obtain a solid assembly. The bonding between the two components can be broken by the application of a sufficient force. The two components can be assembled and separated several times without a significant decrease in the strength of the assembly.
In the specific case of disposable diapers, these fixing systems are arranged on the rear and front parts forming the belt of the diaper when it is worn by the user (see U.S, Pat. No. 5,176,670).
The component containing the loops is fixed to the front part of the diaper, the loops being directed outwards. This component is generally composed of a thin material based on polyolefin fibers or on other synthetic fibers, in particular a knitted fabric (also known as textile mesh). The component is generally provided in the form of a strip of variable size, optionally decorated with patterns, which is adhesively bonded to the outer covering of the diaper. This covering can consist of a film which is impermeable to liquids, for example a polyethylene or polypropylene film, or of a non-woven which is impermeable to liquids and permeable to vapor.
The adhesive bonding of the component containing the loops to the impermeable material can be carried out by means of thermoplastic adhesives of the polyolefin or urethane type (see CA-A 2,122,942) or of moisture-crosslinkable polyurethane adhesives.
The adhesive bonding is carried out industrially at a high rate, that is to say at a speed which can exceed 150 meters per minute. The adhesive-bonding devices operate by coating the impermeable material with the liquid adhesive and laminating the component containing the loops. The assembly, thus formed, is subsequently subjected to calendering and to winding off.
The amount of adhesive deposited must be carefully controlled: an amount which is too low does not make possible satisfactory adhesive bonding and an amount which is too high passes through the pores of the laminated component, with the risk of spreading over the loops, which is harmful to the fixing of the component containing the hooks.
It is economically advantageous to seek to limit the amount of adhesive while maintaining the performance of the adhesive bonding. To this end, the Applicant Company has sought to deposit the adhesive on the component containing the loops.
Under the abovementioned industrial conditions, the adhesives of the prior art have a tendency very quickly to no longer adhere to the component, which results in the adhesive wicking up on the dispensing device and the production line shutting down.
As regards adhesives based on moisture-crosslinkable polyurethanes, an additional constraint is imposed by the level of residual isocyanate monomer, which must be as low as possible for obvious toxicity reasons.
The Applicant Company has now found a novel adhesive which makes it possible to overcome these difficulties.
The invention relates to a moisture-crosslinkable polyurethane adhesive which comprises:
a) 70 to 95% by weight of a polyurethane prepolymer obtained by polyaddition of at least one polyol to at least one diisocyanate trimerized as isocyanurate and,
b) 5 to 30% by weight of an essentially amorphous (poly-xcex1-olefin, the content of free NCO groups representing 1 to 20% by weight of the adhesive.
The moisture-crosslinkable polyurethane adhesive preferably comprises:
a) 85 to 95% by weight of a polyurethane prepolymer obtained by polyaddition of at least one polyol to at least one diisocyanate trimerized as isocyanurate and,
b) 5 to 15% by weight of an essentially amorphous polyolefin, the content of free NCO groups representing 1 to 10% by weight of the adhesive.
The polyol participating in the formation of the prepolymer is generally chosen from polyether polyols, polyester polyols and unsaturated polyols.
The polyether polyols are generally chosen from aliphatic and aromatic polyether polyols and mixtures of these compounds. Their average molecular mass is preferably between 200 and 9000 and their hydroxyl functionality is preferably between 2 and 4.6.
Mention may be made, as examples of aliphatic polyether polyols, of oxyalkylated derivatives of diols, such as polypropylene glycols, or of triols, such as glycerol, trimethylolpropane and hexane-1,2,6-triol, polymers of ethylene, propylene or butylene oxide, copolymers of ethylene oxide and of propylene oxide, the above-mentioned compounds containing silanyl endings, and oxyalkylated diphenyl derivatives, such as derivatives oxyethylenated or oxypropylenated in the 4,4xe2x80x2-position of diphenylmethane.
Use is preferably made of oxypropylated derivatives of glycerol, polymers of propylene or butylene oxide, and copolymers of ethylene oxide and of propylene oxide.
The polyester polyols are generally chosen from aliphatic and aromatic polyester polyols and mixtures of these compounds. Their average molecular mass is preferably between 250 and 7000 and their hydroxyl functionality is preferably between 2 and 3.
Mention may be made, by way of examples, of the polyester polyols resulting from the condensation of aliphatic, cyclic or aromatic polyols, such as ethanediol, 1,2-propanediol, 1,3-propanediol, glycerol, trimethylolpropane, 1,6-hexanediol, 1,2,6-hexanetriol, butenediol, sucrose, glucose, sorbitol, pentaerythritol, mannitol, triethanolamine, N-methyldimethanolamine and mixtures of these compounds, with an acid, such as 1,6-hexanedioic acid, dodecanedioic acid, azelaic acid, sebacic acid, adipic acid, 1,18-octadecanedioic acid, phthalic acid, succinic acid and mixtures of these acids, an unsaturated anhydride, such as maleic or phthalic anhydride, or a lactone, such as caprolactone.
Use is preferably made of the polyester polyols resulting from the condensation of ethanediol, 1,3-propanediol and/or 1,6-hexanediol with adipic acid and/or phthalic acid.
The unsaturated polyols are generally chosen from polyols and mixtures of polyols preferably having a molecular mass of between 1200 and 3000.
Mention may be made, by way of examples, of polybutadiene and polyisoprene containing hydroxylated endings.
The abovementioned polyols are advantageously amorphous.
The abovementioned polyols can also be used as a mixture with other hydroxylated compounds.
It is thus possible to use a monol or a mixture of monols, in particular poly(ethylene/butylene)monols, such as Kraton Liquid(trademark) L-1203 Polymer, sold by Shell, or a polyol or a mixture of polyols chosen from poly(ethylene/butylene)diols, such as Kraton Liquid(trademark) Polymer HPVM-2203, sold by Shell, copolymers of ethylene, of vinyl acetate and of 2-hydroxyethyl acrylate, such as Orevac(copyright) 9402, sold by Elf Atochem, indene/coumarone resins modified by phenol, for example Novares CA120, sold by VFT, and hydroxylated tackifying resins, for example Reagem, sold by DRT.
The abovementioned monol and the abovementioned polyol respectively represent 0 to 10% and 0 to 25% by weight of the polyols.
The diisocyanate trimerized as isocyanurate derives from diisocyanates chosen from the group composed of diphenylmethane 4,4xe2x80x2-diisocyanate, diphenylmethane 2,4xe2x80x2-diisocyanate and hydrogenation products of the abovementioned diisocyanates, toluylene 2,4-diisocyanate, toluylene 2,6-diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate and naphthylene 1,5-diisocyanate and their mixtures.
A trimerized hexamethylene diisocyanate or a trimerized isophorone diisocyanate is preferred.
The diisocyanate trimerized as isocyanurate generally contains less than 0.2% by weight of free isocyanate monomers.
The polyurethane prepolymer is obtained by polyaddition of at least one polyol and of at least one diisocyanate trimerized as isocyanurate in amounts such that the content of isocyanate groups in the prepolymer is between 1 and 25% and preferably between 2 and 15% by weight.
The nature of the polyurethane prepolymer is capable of influencing the hardness of the adhesive according to the invention. In this respect, preference is given to the polyurethane prepolymers obtained by reaction:
of a mixture of polyols composed:
of 20 to 100% by weight of an aliphatic polyester polyol,
and of 0 to 80% by weight of an aliphatic/aromatic polyester polyol
and of at least one diisocyanate trimerized as isocyanurate.
Preference is particularly given to the polyurethane prepolymers obtained by reaction:
of a mixture of polyols composed:
of 30 to 50% by weight of an aliphatic polyester polyol,
and of 70 to 50% by weight of an aliphatic/aromatic polyester polyol
and of at least one diisocyanate trimerized as isocyanurate.
The essentially amorphous poly-xcex1-olefin (APAO) is generally chosen from polymers resulting from the copolymerization of monomers chosen from ethylene, propene, 1-butene and 1-hexene in the presence of a catalyst of Ziegler-Natta type and mixtures of these copolymers. Mention may be made, by way of examples, of Vestoplast(copyright), in particular referenced 508, sold by Hxc3xcls, Eastoflex(copyright), sold by Eastman Chemical, and Rextac(copyright), sold by Rexen. Preference is given to copolymers having a molecular mass of between 7300 and 23,800 and a glass transition temperature of between xe2x88x9227 and xe2x88x9236xc2x0 C. Use is advantageously made of copolymers having a ring-and-ball softening point (DIN Standard 52011) of between 80 and 160xc2x0 C.
The moisture-crosslinkable adhesives according to the invention can comprise conventional additives known to a person skilled in the art, such as fillers, tackifying agents, plasticizers, adhesion promoters, such as compounds containing silanyl groups, and catalysts which accelerate the crosslinking by moisture, for example metal catalysts, such as dibutyltin dilaurate, or amine-containing catalysts, such as dimorpholinodiethyl ether.
The moisture-crosslinkable polyurethane adhesives according to the invention can be prepared by reacting the polyols and, if appropriate, the other hydroxylated compounds, optionally dehydrated at high temperature under vacuum, with the isocyanurates with the exclusion of moisture and, if appropriate, under a protective gas, at a temperature which can vary from 90 to 120xc2x0 C. After the initiation of the reaction, the poly-xcex1-olefin is added, by mixing, in the desired quantitative ratios and the reaction is allowed to continue until the desired NCO content is obtained. The optional additives are added, by mixing, to the adhesive thus obtained.
The moisture-crosslinkable polyurethane adhesives according to the invention are particularly suited to the industrial adhesive bonding of textile meshes, for example made of cellulose, of polyamide or of poly(ethylene terephthalate), and of plastic films, for example made of polyethylene, of polypropylene or of poly(ethylene terephthalate). This industrial adhesive bonding is generally employed at a high rate, that is to say with a speed of progression of the mesh which can exceed 150 meters per minute, or indeed more.
The adhesives according to the invention additionally exhibit a content of free isocyanate monomer of less than 0.15% by weight of the adhesive, a low vapor pressure at the temperature of use (100 to 130xc2x0 C.) and consequently a limited toxic nature, an excellent trapping power (ability of adhesive to fix a material) and a good elasticity after crosslinking by moisture.
The following examples allow the invention to be illustrated.